Daisy chaining is the simplest way to connect a network. Devices connected by a daisy chain are connected to one to another in series and a message that is sent on the network has to travel down the chain from one device to another. Compared to other network topologies, daisy chaining is relatively slow, however in applications that do not require large amounts of data transfer and fast transfer rates, daisy chaining is still a common practice and daisy chain networks are still very common in industrial control networks.
One common standard that uses a daisy chain configuration for networking devices is the RS-485 standard. While RS-485 devices may be quite common, there are other protocols that specify or can use a daisy chain network configuration such as Apple's LocalTalk™ and many types of industrial applications.
While networked devices using the RS-485 protocol have always been common in industrial systems, such as larger scale heat and ventilation systems, with the decrease in price of control systems, smaller scale control systems are becoming more common. One area using networked devices that can use a daisy chain topology is home automation and especially home HVAC systems.
In order to setup devices in a daisy chain network, a cable has to be strung to each of the devices in the network. With the exception in some cases of the first and last devices in a daisy chain network, each device in the network requires a cable running to it from a previous device and another cable running from it to the next device. This requires each device connected to a daisy chain network to have a first connection for an input cable, coming from the previous device in the network, to be connected to each network device and a second connection for an output cable, running to the next device in the network.
This configuration is necessary because daisy chaining connections utilize termination resistors on each end of the network to ensure that every transceiver is directly connected to the main current path. Transceivers placed outside the termination resistors daisy chain may not be able to correctly sense the voltage drop and “hear” the transmission. In this way “star” wiring configurations are not allowed for daisy chained networks such as RS-485.
The disadvantage of wiring the network is this fashion is that there must be some overall plan to the creation of the network. The devices must be planned to some degree because a cable running from the previous device must be connected to the device and then a second cable connected to the same device and then a second cable connected in another port of the same network device must be run to the next device. This requires the person setting up the network to know where the previous device is as well as the location of the next device. Knowing the placements of the devices may not be overly complicated when the network is small and centralized in one area, but often these daisy chain networks have long distances between devices and these devices might be in different locations that are not in sight of each other. For example, in a HVAC system for a house, the devices connected to the chain network will typically be a controller near the furnace and a number of thermostats connected to the daisy chain network and spread throughout the house. Each thermostat device connected to the network will likely be situated in a different room or location of the house from other devices and it will not always be easy to determine in which direction to run the cable to and from each device.
Additionally, some of the protocols such as RS-485 networks require a termination resistor at the end of the network. This requires one of the devices to serve as the last device and the network must be planned to end at the device that has the termination resistor in it.
Not only must the daisy chain network be planned to some degree, but it can also be complicated to add new devices to the network. To add a new device, the network must be disconnected from one of the device and the new device incorporated into the chain. Again, the location of the previous device and next device must be known, which might not be that easy to determine if the network is spread throughout a large building and numerous rooms.
The different standards for daisy chain networks also specify the type of cable that is required in order to connect the devices. RS-485, for example, specifies certain minimum standards for cable and requires the cable to be a twisted pair in order to use balanced differential signals to reduce or eliminate the effect of interference in the cables.
There are many cables available that meet the recommendations for the different daisy chain network protocols and there are cables that are specially designed for use with these applications. These cables are quite specialized and although daisy chain networks are common, they are not as common as other more standard types of networks. This often makes the special cabling more costly and harder to find because of its lower production. Also, electricians are often not familiar with these types of specialty cables.
In more recent years a number of more standard cable specifications have arisen that are not specifically made for daisy chain networks. One very common type of standard cable is referred to as Category 5 cabling. These standardized cables often include a number of conductors or wire strands and standardized connections to increase the ability of these standard cables to be used in a number of different applications i.e. category 5 consists of four twisted pairs of copper wire terminated by RJ45 connectors.
Because these standard cables can be used in so many applications and circumstances and some, like Category 5 wire, are in common use, they are manufactured in very large quantities which often makes them cheaper then other specialty cables, easier to find and electricians and other installers are more often more familiar with their use.